The present invention relates generally to the field of explosives and more particularly to means, known as detonators, used to detonate shaped charges of a perforating gun. More particularly, the present invention relates to a capacitor exploding foil initiator device for detonating shaped charges of a perforating gun.
A common method of completing a well is to set a casing through the oil-bearing formation and cement it. In this well known method, after the casing string is run, cement is pumped down the inside of the casing. The cement flows through the bottom opening in the casing and starts up the annular space between the outside of the casing and the wall of the well (or bore hole). Pumping of the cement continues until the cement fills the annular space. Then a plug is forced down the inside of the casing by a displacement fluid (e.g., salt water) and the cement is allowed to harden. Once the well has been completed, as described above, access to the oil-bearing portion of the formation must be provided, typically by perforating the casing and the cement at selected locations. A perforating gun is commonly employed to provide these perforations. The perforating gun is device fitted with shaped charges or bullets that is lowered into the inside of the casing at a selected depth and fired to create penetrating holes (i.e., perforations) in the casing, cement and formation. The formation fluids (e.g., oil) flow out of the reservoir therein through the perforations and into casing.
Detonation of these shaped charges (i.e., a secondary explosive) is initiated by devices using a primary explosive. Such devices include: detonators (e.g., blasting caps); exploding bridge wire devices; and resistor bridges.
It is well known that the passage of an electric current through a conductor generates a certain amount of heat, the amount of heat varying directly with the resistance of the conductor and with the square of the current. This phenomenon is relied upon in fusible links that are installed in electrical circuits to prevent the flow of more than a predetermined amount of current in such a circuit. When the predetermined flow is exceeded, the heat melts the fusible link so that the circuit is broken. If a sufficient current is passed through the link in a small period of time, the link is not only melted but may be vaporized. If the fusible link is enclosed in a small space the vaporizing of the link can increase the pressure in that space.
The blasting caps include a heat sensitive primary explosive set off by an electrical resistance heated by the passage of an electric current through the resistance. The exploding bridge wire devices detonate a primary explosive using a relatively low resistance bridge extending between conductors and through which a relatively high current is passed so that the bridge portion is not only heated to its melting point but is heated so much that it vaporizes and literally explodes to provide a shock wave to detonate the primary explosive. While such a system can use a primary explosive that is much less sensitive to heat and shock than a secondary explosive, there are still a distressing number of accidents that occur when the primary explosive is prematurely detonated. While less sensitive explosives have heretofore been available, it has been difficult to cause the detonation of such explosives at a selected time.
Recently, it has been proposed to detonate these more stable explosives by an electrical means of some sort that creates a sudden pressure to shear a film and form a disk or flyer which is then impacted against the explosive material.
In the construction of such a detonator, it is important that the explosive material be properly supported and sealed against the admission of materials such as moisture that would tend to deteriorate it. This is particularly important when the detonator is used in environments, such as deep wells, where the ambient pressures can become very high. It is also important that the physical construction be such that the flyer has sufficient kinetic energy imparted to it to insure the detonation of the explosive.
One such example is disclosed in U.S. Pat. No. 4,602,565 (which is incorporated herein by reference) wherein an exploding foil detonator uses an explosive that is detonated by a flyer that is sheared form a sheet or film and propelled through a barrel to impact the explosive. The flyer is sheared from the sheet by the pressure generated when an electrical conductor adjacent the sheet is vaporized by the sudden passage of a high current (as by the discharge of a capacitor) through it. The explosive is sealed against moisture, and the mechanical configuration of the detonator is such as to take full advantage of the kinetic energy of the flyer.
As is typical in the prior art, the capacitor is in a circuit with the foil detonator and a normally open switch. When it is desired to arm the system, the capacitor is charged, e.g., to 3000 volts; when it is desired to initiate the explosion, the switch is closed and the capacitor discharges through the foil vaporizing the same. A high resistance bleed resistor connected across the capacitor is used to bleed off the charge on the capacitor in the event that the latter is charged but then not discharged into the load.
Premature explosion of the secondary explosive has occurred as a result of unintended detonation of the primary explosive (used to detonate the secondary explosive) resulting in loss of life and equipment. Accordingly, a need exist for a more reliable means for initiating the secondary explosive.